CODEWITHSUNDEEP
openmp
xuhdev
xuhdev

Reputation: 9343

OpenMP: sum reduction -- incorrect result if not using +=

I have the code as following:

#include <stdio.h>
#include <omp.h>

#define N       10
double x[N];
int main(void)
{
    double sum = 0.0;
#pragma omp parallel
    {
#pragma omp for
        for (int i = 0; i < N; ++ i)
            x[i] = (double) i;

#pragma omp for reduction(+:sum)
        for (int i = 0; i < N; ++ i)
        {
            sum += (double) x[i];
        }
    }

    printf("%le\n", sum);

    return 0;
}

The output is 45, which makes sense. However, if I replace sum += (double) x[i] with sum = (double) x[i], the output is then always 43. Could anyone explain why this happens?

Thanks!

Upvotes: 0

Views: 725

Answers (2)

xuhdev
xuhdev

Reputation: 9343

I think I know what's happening here. I thought sum was copied for each ITERATION, but actually sum is copied for each THREAD. I have 8 threads running, so one of them has lost its initial value. Solved. Thanks for all your answers here!

Upvotes: 1

Alejandro Alcalde
Alejandro Alcalde

Reputation: 6220

If you replace sum += (double) x[i] by sum = (double) x[i], you are not acummulating the result, hence, sum will store the value of the last iteration.

Upvotes: 3

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